This invention relates to a private branch exchange and, more particularly, to a private branch exchange in which an extension accommodates an ISDN (integrated services digital network) terminal and a call transfer service or voice-recording service is supported via (or from) the ISDN terminal.
Additional services have been proposed for use in a network service that employs a high-speed information communication service such as an INS (information network system) serviced by NTT (Nippon Telegraph and Telephone Corporation). One example of such an additional service is a call transfer function. According to this function, an incoming call is not answered directly but is transferred to another user. One major advantage of this service is that since the service is provided on the INS side, one's own exchange does not require extra memory or hardware even though this service is enjoyed.
In order to utilize this call transfer function, a limitation is imposed in a case where the INS of NTT is used. Specifically, an ISDN terminal that the additional service function is capable of utilizing must be connected to an S/T point (i.e., S or T point) and a T point among user-network interface reference points (in line with JT-I411 of the Telecommunication Technology Committee (TTC) standards), where an S point is defined as a connection point in a network interface with a terminal, and a T point is defined as a connection point in a network interface with a network. Further, an agreement (contract) for the purpose of utilizing the additional service is required between the user and the network.
On the other hand, in a case where an ISDN terminal is used as an extension terminal of an exchange, this terminal must be connected to an S point rather than the aforementioned S/T point and T point of the user-network interface reference points. As a consequence, one must accept the inconvenience of not being able to utilize an additional service function such as the call transfer function.
If use of an ISDN terminal as the extension terminal of an exchange cannot be avoided and, moreover, it is desired to be able to use the transfer function as well, then, as a last resort, the user employing the conventional exchange must take the trouble to perform an "absent" setting with regard to his own terminal, thereby making this terminal an absence terminal, and must instead register the transfer-addressed extension number or an outside-line number with the exchange. If there is an incoming call, this operation causes the call to be transferred to the registered transfer address. In other words, the user does nothing more than employ an absence transfer function as the call transfer function in pseudo-fashion.
With the conventional exchange mentioned above, however, the following difficulties are encountered when using the call transfer function:
(1) Since a service function on the Information Network System ("INS") side is performed in so-called pseudo-fashion within one's own exchange, a memory incorporated within the extension is used to register the transfer-addressed extension number and the outside-line number. As a consequence, the internal memory for the absence setting must be increased in proportion to an increase in the number of extensions accommodated.
(2) It is difficult with the pseudo-transfer function to register a transfer destination if the transfer destination cannot be established at one location. As a result, the transfer service cannot be utilized effectively.
Further, owing to the fact that the ISDN terminal must be accommodated in the extension (S point) of the exchange, the large number of ISDN terminal functions which would be obtained if the ISDN terminal were connected to the S/T point or T point are not realized.
Still another disadvantage of using an ISDN terminal as an extension terminal of a private branch exchange occurs when a so-called "shift call" (or "reset call") operation is performed between extension terminals.
In a case where a transmission is made from an ISDN terminal of an extension to another extension terminal in a private branch exchange (PBX) according to the prior art, the extension number of only one party can be designated to make the transmission; transmissions cannot be made by designating a plurality of extension numbers. Accordingly, when the calling party calls an extension number from an ISDN terminal and the called party is found to be busy (a state in which the incoming call cannot be received), the calling party is required to temporarily release the call upon hearing a busy tone (BT) and then call another extension again. As long as the other party is busy again, the same operation must be repeated many times. This operation must be repeated until the party that is capable of receiving the incoming call receives the call. Though a shift call function from a single line telephone or leased telephone can be utilized in the conventional PBX (Private Branch Exchange), this function cannot be utilized from an ISDN terminal.
In a case where a call is placed from an extension telephone and the called party is busy, the shift call function is such that the call is transmitted automatically to an extension telephone having a number only the last digit of which differs from the extension number of the called party. In order to use this function, a call is issued from the telephone of an extension and, when the called party is busy, this party transmits a busy tone (BT) from the private branch exchange (PBX) to the telephone on the calling side. During the transmission of BT, the user on the calling side is required to dial in not only the extension number of the called party, but the last digits (one digit other than "1" if the extension number is "301") of extension numbers that differ from the extension number of the called party. When this is done, the PBX, which has received the dialed numeral, substitutes the received numeral for the last digit of the extension number of the earlier party. If the extension having the number with the substituted last digit is capable of receiving the incoming call, then this extension is made to receive the call.
The reason why the "shift call" function is capable of being utilized by a single line telephone in the prior art but is incapable of being utilized by an ISDN terminal telephone will be described in detail with reference to FIGS. 1 through 3.
FIG. 1 is a block diagram for describing the conventional PBX. As shown in FIG. 1, a PBX 501 accommodates, as extension terminals, an ISDN terminal 502 having extension number "201", single line telephones (SLT) 503-505 having extension numbers "301"-"303", respectively, and a single line telephone (SLT) 513 capable of utilizing the shift control function of the PBX 501. The SLT 513 is not an ISDN terminal.
The PBX 501 has an ISDN extension interface (i.e., basic rate interface or BRI) 506 for interfacing the ISDN terminal 502, SLT extension interfaces (SLTI) 507-509 for interfacing the SLT's 503-505, respectively, a communication controller (CC) 510 for supervising overall control of the PBX 501, a main memory (MM) 511 for storing various programs and data, a channel switch (TSW) 512 for connecting a voice channel when an extension conversation is performed, and an SLT extension interface (SLTI) 514 for interfacing the SLT 513.
The BRI 506 has call detecting means for detecting origination of a call from the ISDN terminal 502. The controller 510 has called number retrieving means for retrieving information, which designates the receiving party from the set-up message, when a call originates from the ISDN 504, called number analyzing means for analyzing the called extension from the information retrieved, and incoming-call control means for controlling the incoming call to the called extension. The memory 511 has a call-status storage table for storing the status of extensions.
FIGS. 2 and 3 are flowcharts illustrating a procedure for a case in which an outgoing-call transmission is made from the ISDN terminal 502, which does not possess the shift call function of the prior art, to another extension.
First, in a case where an outgoing call is sent from the ISDN terminal 502 of the extension number "201" to the extension number "301" (step S601), the BRI 506 in the PBX 501 detects this outgoing call and so notifies the controller 510 (step S602).
Upon receiving notification of the outgoing call, the controller 510 retrieves the information, which designates the number of the called party, from the set-up message of this call and extracts the information (step S603). It should be noted that this information is contained in a "call number" or "keypad facility" information element.
Upon extracting the party-number information, the controller 510 analyzes the called extension from this information (step S604) and judges that the called extension is the SLT 503 of extension number "301".
Next, the controller 510 examines the call status of extension number "301" by referring to the call-status storage table within the memory 511 (step S605). If extension number "301" is idle and capable of receiving the incoming call (YES at step S606), then the controller 510 shifts the area corresponding to the extension number "301" in the call-status storage table of memory 511 from the idle state to the busy state (step S607) and controls the SLTI 507 of extension number "301" so that the SLT 502 of extension "301" will receive the incoming call (step S608).
While the call is being received by the SLT 503 and the party is being called, the PBX 501 causes the ISDN terminal 502 to issue an RBT (ring-back tone).
If the SLT 503 answers (YES at step S610), this is detected by the SLTI 507, which proceeds to notify the controller 510. Upon being notified of the answer, the CC 510 controls the TSW 512 so as to connect a voice channel between the extension number "201" and the extension number "301", whereby a transition is made to a conversational state (step S611).
In a case where the SLT 503 fails to respond within a certain period of time (NO at step S610), the ISDN terminal 502 determines that the party is in a state in which it cannot answer and releases the call (step S609). In this case, the controller 510 shifts the area corresponding to the extension number "301" in the call-status storage table of memory 511 from the busy state to the idle state (step S612), halts the incoming call to the called extension (step S613) and releases the call (step S614).
If the extension "301" is found to be incapable of receiving the incoming call at step S606 because it is busy (or for some other reason), then the BRI 506 is controlled so as to send BT (a busy tone) to the ISDN terminal 502 of extension "201" and refuse the call (step S615). As a result, the user of the ISDN terminal 502 hears the BT and releases the call (step S616).
In a case where the extension "301" is incapable of participating in the call in the operation described above, the ISDN terminal 502 sends the outgoing call to the extension number "302", which is in the vicinity of extension number "301". It should be noted that the operation that follows the origination of the call is the same as that described in the case of extension "301" above. If the extension number "302" also is incapable of taking part in the call, then the ISDN terminal 502 sends the outgoing call to extension number "303", which is in the vicinity of the extension numbers "301", "302".
Thus, in a case where it is desired to establish communication with a certain party from an ISDN terminal with the conventional PBX, the call originating operation is repeated a number of times until the call can be transmitted to the extension telephone at which the party is present.
Next, a procedure will be described for a case in which an outgoing-call transmission is made from the SLT 513 (not an ISDN terminal) terminal 502, which possesses the shift call function of the prior art, to another extension.
First, in a case where an outgoing call is sent from the SLT 513 of the extension number "202" to the extension number "301", the SLTI 514 notifies the controller 510 of the dialed numerals received from the SLT 513. Upon receiving notification of reception of the dialed numerals, the controller 510 analyzes the dialed numerals and determines that the incoming call is for the SLT 503 of extension number "301".
Next, the controller 510 examines the call status of extension number "301" by referring to the call-status storage table within the memory 511. If it is discovered that the extension number "301" is busy and is incapable of receiving the incoming call, then BT is sent to the SLT 513 of extension number "202".
Upon hearing the BT, the user of SLT 513 enters the dial number "2", while still listening to the BT, in order to request transmission of the call to extension number "302", which is in the vicinity of extension number "301".
As a result, when the SLTI 514 receives the dialed numeral "2", the controller 510 is notified of this numeral. Upon being so notified, the controller 510 analyzes this numeral and determines that the destination of the incoming call is SLT 504 of extension number "302". Then, upon referring to the call status of the extension "302" in the call-status storage table within memory 511 to determine that the extension "302" is in the idle state and is capable of receiving the incoming call, the controller 510 shifts the area corresponding to extension number "302" in the call-status storage table of the memory 511 from the idle state to the busy state and controls the SLTI 508 corresponding to extension "302" so that the incoming call is received at the SLT 504 of extension "302".
While the call is being received by the SLT 504 and the party is being called, the PBX 501 causes the SLT 513 to issue the Ring Back Tone ("RBT").
If the SLT 504 fails to respond within a certain period of time, the user of SLT 513 enters the dial number "3", while listening to the RBT, in order to request transmission of the call to extension number "303", which is in the vicinity of extension numbers "301", "302", and requests transmission of the call to number "303", which is the result of substituting the numeral "3" for the last digit of earlier call destination "302".
Henceforth the call-originating operation utilizing the shift call function is repeated until communication with the desired party is established by the procedure set forth above.
When an answer is received from the party, a transition is made to the conversational state through an operation similar to that for the case of the ISDN terminal 502.
(3) Thus, another disadvantage of the conventional PBX is that since the shift call function cannot be utilized when it is attempted to send a call from the ISDN terminal of an extension to another extension, it is necessary to release the call and re-dial a neighboring extension whenever the desired party is busy or cannot answer. This is a very troublesome task.